Triangles form one-way channels

California Institute of Technology researchers
have designed a microfluidic rectifier that is simply a channel whose
shape makes flow resistance different for fluids flowing in opposite directions.

This makes it act like a diode, which allows electricity to flow
in only one direction, or a mechanical check valve, which blocks fluids
from reversing direction.

Flow in the reverse direction faces more than twice the resistance
of forward flow.

The microfluidic rectifier could be used in integrated microfluidic
circuits, which use control fluids to operate pumps and valves that move
samples and reagents in biochips.

The active element of the device is a chain of 43 triangles running
down the middle of the channel. The channel is 100 microns deep.The triangles
are 230 microns from base to tip, and 330 microns wide. A micron is one
thousandth of a millimeter.

Key to the achievement is the type of fluid the researchers used
-- a viscoelastic polymer that has long, chain-like molecules that entwine
like strands of thread. The molecules stretch as the fluid passes through
a constriction, and eventually the molecules unravel, which increases
the fluid's flow resistance at a greater rate than the increase in applied
pressure.

One advantage of the researchers' simple design is that it relatively
easy to fabricate. The microfluidic rectifier is a single mold sealed
with a glass cover.

The method could be implemented within a year, according to the
researchers. The work is scheduled to appear in an upcoming issue of Physical
Review Letters.